1. Field of the Invention
The present invention relates to battery chargers, and in particular, to a battery charger controller capable of charging secondary batteries that have been deeply discharged as low as 0V.
2. Discussion of the Related Art
In a battery charging system, a current control (CC) circuit and a voltage control (VC) circuit work in sequence to recharge a discharged battery. During bulk charging operation, the CC circuit provides a constant rapid-charge current to rapidly charge the battery until a final target voltage is reached. At that point, the charging system switches to maintenance operation as the VC circuit takes over to maintain the battery voltage at the final target voltage. The rapid-charge current applied by the CC circuit provides the maximum battery charging rate and reduces the overall recharging cycle time. However, a deeply discharged battery having a voltage below a minimum specified threshold voltage cannot accept the high rapid-charge current without risk of storage capacity degradation. The recommended method for charging a deeply discharged battery is to recharge at a reduced rate by providing a lower conditioning current until the battery voltage is above the minimum threshold voltage. This type of charging is referred to as hi-Z mode charging or battery conditioning charging.
In a conventional battery charging system with hi-Z charging capability, the conditioning current control is provided by a modified CC circuit. A conventional scheme for providing hi-Z charging is depicted in FIG. 1. A power source 101, controlled by a CC circuit 103 and a VC circuit 104, provides a charging voltage Vs to a battery 105. During bulk charging operation, CC circuit 103 monitors a charge current Ibatt flowing through battery 105 and maintains it at a rapid-charge current Imax. When a voltage Vbatt, measured across battery 105 by a differential amplifier 106, reaches a final target voltage Vfinal, VC circuit 104 maintains voltage Vbatt at voltage Vfinal. An output driver circuit 102 determines whether CC circuit 103 or VC circuit 104 has control of the battery charging system, and sends the appropriate control signal Vc to power source 101. CC circuit 103 includes a reference voltage generator 108 and an error amplifier 109. During bulk charging operation, reference voltage generator 108 produces a voltage Vref equal to a voltage Vrefa. Voltage Vrefa is defined by the formula: EQU Vrefa=Imax*R114
where R114 is the resistance of a monitoring resistor 114. Therefore, as it attempts to keep its inputs equal, error amplifier 109 maintains a constant current Imax flowing through battery 105. However, if voltage Vbatt is less than a minimum voltage Vmin, current Imax can permanently degrade the storage capacity of battery 105, so hi-Z mode charging must be performed. A hi-Z control circuit 107 detects when hi-Z charging is required and ensures that proper charging takes place. An embodiment of hi-Z control circuit 107 includes comparators 110 and 111, an AND gate 112, and a fault circuit 113. Comparator 110 compares voltage Vbatt to a reference voltage Vlco. Voltage Vlco is the minimum battery voltage at which the charging system can properly function. If voltage Vbatt is less than voltage Vlco, comparator 110 generates a logic LOW output signal, causing fault circuit 113 to assert a Vfault signal to prevent any charging operation. While voltage Vbatt is greater than voltage Vlco but less than voltage Vmin, comparator 110 outputs a logic HIGH signal while comparator 111 asserts a logic LOW signal. As a result, AND gate 112 sends a logic LOW signal to reference voltage generator 108, which generates a voltage Vrefb as its output voltage. Voltage Vrefb is defined by the formula: EQU Vrefb=Icond*R114
where Icond is a conditioning current required for proper hi-Z charging of battery 105. Thus, while Vbatt is less than Vmin, error amplifier 109 forces voltage Vs lower and lower until current Ibatt equals current Icond. When voltage Vbatt reaches voltage Vmin, the output of comparator 111 swings to a logic HIGH stage, bringing the output of AND gate 112 HIGH. This in turn switches the output of reference voltage generator 108 back to voltage Vrefa, which raises the charge current to Imax and begins bulk charging operation.
Due to the control system used in the aforementioned hi-Z charging circuit, a battery that has been discharged below voltage Vlco cannot be recharged. Such a deeply discharged battery begins to approximate a short circuit, and cannot be properly handled by conventional charging systems.
While some charging systems have overcome the limitation of recharging a deeply discharged battery, these charging systems have other shortcomings. An example of this type of charging system is battery charging IC's bq2031 and bq2054 from Benchmarq, which provide hi-Z charging for lead-acid and lithium-ion batteries, respectively. While these battery chargers allow charging of deeply discharged battery, these systems require a separate power supply to power the charger circuitry. The need for a separate power supply arises because hi-Z charging is provided by the reduction of source voltage Vs until current Ibatt drops to current Icond. During hi-Z charging operation, voltage Vs will fall below a rated operating supply voltage required by other charger circuits, such as circuits 102, 103, 104, 106, and 107 in FIG. 1. Therefore, an independent, fixed voltage source must be used to provide the rated operating supply voltage for these charger circuits.
Accordingly, it is desirable to provide a hi-Z charging control circuit that is capable of charging a battery having a voltage as low as 0 V and does not require a separate supply voltage for related circuitry.